Head mounted listening devices, such as hearing aids and headsets or similar devices, have been developed in recent years. In hearing aids for instance in an “In-The-Ear”(ITE) or in an “Behind-The-Ear” (BTE) application, an input audio signal is processed through signal processing, and is transmitted to the user of the hearing aid.
In listening devices, the signal processing should result in improvements in speech intelligibility and sound quality. Typically, tradeoffs between size, power consumption and noise are made by the listening device designer as part of their design process. Designers want more processing capability (which is proportional to power consumption) and the smallest size possible. Once a designer has determined an acceptable size and power consumption, the noise level (either tonal or stochastic) must be addressed. If designers push the size and/or power consumption parameters too far, undesired audible side effects (artifacts) on the output of the listening devices, in the form of tonal or stochastic noise, may result.
Currently available listening devices usually contain an audio subsystem (such as amplification units, aliasing filtering units, analog-to-digital (A/D) conversion units, digital-to-analog (D/A) conversion units, a receiver, a loudspeaker), and a plurality of subsystems, each of which performs signal processing.
For instance, consider a listening device system that contains one or more victim subsystems (Vx1, Vx2, . . . ) and one or more attacker subsystem (Ay1, Ay2, . . . ). The listening device system may contain one or more others subsystem (Oz). All the subsystems are connected to a common power supply (P). The common power supply (P) provides a voltage (U) and a current (I) to the listening device system. The victim subsystems (Yx1, Vx2, . . . )are characterized as sensitive to a variation in the voltage (U) of the common power supply (P). The attacker subsystems (Ay1, Ay2, . . . ) are characterized as consuming a dynamic current (dIy) through the common power supply (P). The dynamic current (dIy) is periodic with a period (Ty). The other subsystems (Oz) are characterized as non-sensitive to a variation of the power supply voltage and are not consuming a periodic dynamic current through the common power supply (P). A subsystem could be a victim and an attacker. Each dynamic current (dIy) produces a variation of the voltage (U) of the common power supply (P) equal to the internal resistor of the power supply (Rs) divided by the dynamic current (dIy). The sum of the periodic dynamic current (dIy) produces a voltage variation (dU) of the power supply (P). The spectrum of the voltage variation (dU) is the resulting power supply noise (SN). The audible power supply noise (AN) is a part of the power supply noise (SN) characterized by the fact that it is in the audio-band of interest (typically 20 Hz to 20 kHz but not limited hereto). Noise is classified as any unwanted or undesired audio signal.
For example, the victim subsystem (Vx1) is an audio subsystem, and the audio subsystem (Vx1) and two aggressor digital subsystems (Ay1, Ay2) are powered by the common power supply (P). The subsystem (Ay1) may process data 2000 times per second while the subsystem (Ay2) may process data packets 32000 times a second. Assume that processing a data packet is associated with drawing current from the common power supply (P), the subsystem (Ay1) draws current 2000 times per second while the subsystem (Ay2) draws current 32000 times per second.
As such, this current which each subsystem draws is dynamic in nature, and may couple into the audio subsystem through the common power supply (P). In this case, the dynamic current draw caused by the subsystem (Ay1) could potentially result in a voltage variation on the common power supply (P) as a result of the dynamic current drawn through the shared output resistance of the common power supply (P). Since the audio subsystem (Vx1) is also powered by the common power supply (P), this voltage variation could potentially propagate through the audio subsystem (Vx1) and therefore also into the audio path causing audible clicks, pops, tones or other undesired audible side effects.
The audible side effects related to dynamic current are often solved by using external, large-size passive-component solutions in the form of capacitors, resistors, and/or inductors, which are applied to power supply voltages going in or out of the subsystems. These passive-component solutions constitute filters that reduce the voltage variations. Depending on the frequency and amplitude of the voltage variations, the filters can require more or larger passive components. However, adding more or larger passive components is not beneficial in a space constrained application like a listening device.
Another solution for resolving the problem is reducing the sensitivity of the victim subsystems to the dynamic current. Here, several techniques are used including (but not limited to): internal power supply filtering in the subsystem, and use of a digital design approach rather than an analog design approach. An internal power supply filter reduces the audible side effects of dynamic current in the same manner as external filters.
It is therefore desirable to provide a method and system, which allows designers to realize small size and computationally capable listening device designs and can reduce audible side effects of dynamic current consumption without the need for large, external solutions as described above.